Radiant energy
From Wikipedia, the free encyclopedia
|
Radiant energy is the energy of electromagnetic waves.[1] The quantity of radiant energy may be calculated by integrating radiant flux (or power) with respect to time and, like all forms of energy, its SI unit is the joule.
[edit] Terminology use and history
The term "radiant energy" is most commonly used in the fields of radiometry, solar energy, heating and lighting, but is also sometimes used in other fields (such as telecommunications). In modern applications involving transmission of power from one location to another, "radiant energy" is sometimes used to refer to the electromagnetic waves themselves, rather than their energy (a property of the waves). In the past, radiant energy has been called "electro-radiant energy".[2] The term "radiant energy" has historically also been applied to electrostatics (in a medium, such as a aether).[3]
[edit] Analysis
Because electromagnetic (EM) radiation can be conceptualized as a stream of photons, radiant energy can be viewed as the energy carried by these photons. Alternatively, EM radiation can be viewed as an electromagnetic wave, which carries energy in its oscillating electric and magnetic fields. These two views are completely equivalent and are reconciled to one another in quantum field theory (see wave-particle duality).
EM radiation can have various frequencies. The bands of frequency present in a given EM signal may be sharply defined, as is seen in atomic spectra, or may be broad, as in blackbody radiation. In the photon picture, the energy carried by each photon is proportional to its frequency. In the wave picture, the energy of a monochromatic wave is proportional to its intensity. This implies that if two EM waves have the same intensity, but different frequencies, the one with the higher frequency "contains" fewer photons, since each photon is more energetic.
When EM waves are absorbed by an object, the energy of the waves is typically converted to heat. This is a very familiar effect, since sunlight warms surfaces that it irradiates. Often this phenomenon is associated particularly with infrared radiation, but any kind of electromagnetic radiation will warm an object that absorbs it. EM waves can also be reflected or scattered, in which case their energy is redirected or redistributed as well.
[edit] Open systems
Radiant energy is one of the mechanisms by which energy can enter or leave an open system. Such a system can be man-made, such as a solar energy collector, or natural, such as the Earth's atmosphere. In geophysics, transparent greenhouse gases trap the sun's radiant energy (at certain wavelengths), allowing it to penetrate deep into the atmosphere or all the way to the Earth's surface, where they are re-emitted as longer wavelength radiation (chiefly infrared radiation). Radiant energy is produced in the sun as a result of nuclear fusion.
[edit] Applications
Radiant energy, as well as convective and conductive energy, is used for heating homes. It can be generated electrically by infrared lamps, or can be absorbed from sunlight and used to heat water. Since radiant energy is really just electromagnetic radiation under another name, it is the basis of a wide range of communication technologies using radiofrequency and microwave radiation.
One of the earliest wireless telephones to be based on radiant energy was invented by Nikola Tesla. The device used transmitters and receivers whose resonances were tuned to the same frequency, allowing communication between them. In 1916, he recounted an experiment he had done in 1896.[4] He recalled that "Whenever I received the effects of a transmitter, one of the simplest ways [to detect the wireless transmissions] was to apply a magnetic field to currents generated in a conductor, and when I did so, the low frequency gave audible notes."
[edit] SI radiometry units
[edit]
| Quantity | Symbol | SI unit | Abbr. | Notes |
|---|---|---|---|---|
| Radiant energy | Q | joule | J | energy |
| Radiant flux | Φ | watt | W | radiant energy per unit time, also called radiant power |
| Radiant intensity | I | watt per steradian | W·sr−1 | power per unit solid angle |
| Radiance | L | watt per steradian per square metre | W·sr−1·m−2 | power per unit solid angle per unit projected source area. Sometimes confusingly called "intensity". |
| Irradiance | E | watt per square metre | W·m−2 | power incident on a surface. Sometimes confusingly called "intensity". |
| Radiant exitance / Radiant emittance | M | watt per square metre | W·m−2 | power emitted from a surface. Sometimes confusingly called "intensity". |
| Spectral radiance | Lλ or Lν |
watt per steradian per metre3 or watt per steradian per square metre per hertz |
W·sr−1·m−3 or W·sr−1·m−2·Hz−1 |
commonly measured in W·sr−1·m−2·nm−1 |
| Spectral irradiance | Eλ or Eν |
watt per metre3 or watt per square metre per hertz |
W·m−3 or W·m−2·Hz−1 |
commonly measured in W·m−2·nm−1 |
[edit] See also
[edit] Main
[edit] Science
- Photoelectric effect
- Zero-point energy
- Open system
- Cosmic microwave background radiation
- Schumann resonance
[edit] Spectrum
| Radio spectrum | ||||||||||
| ELF | SLF | ULF | VLF | LF | MF | HF | VHF | UHF | SHF | EHF |
| 3 Hz | 30 Hz | 300 Hz | 3 kHz | 30 kHz | 300 kHz | 3 MHz | 30 MHz | 300 MHz | 3 GHz | 30 GHz |
| 30 Hz | 300 Hz | 3 kHz | 30 kHz | 300 kHz | 3 MHz | 30 MHz | 300 MHz | 3 GHz | 30 GHz | 300 GHz |
| The Electromagnetic Spectrum | |
|---|---|
|
(Sorted by wavelength, short to long)
Gamma ray • X-ray • Ultraviolet • Visible spectrum • Infrared • Terahertz radiation • Microwave • Radio waves |
|
| Visible (optical) spectrum | Violet • Blue • Green • Yellow • Orange • Red |
| Microwave spectrum | |
| Radio spectrum | EHF • SHF • UHF • VHF • HF • MF • LF • VLF • ULF • SLF • ELF |
| Wavelength designations | |
[edit] References and further reading
[edit] Notes
- ^ "Radiant energy". Federal standard 1037C
- ^ Examples of the term 'electroradiant energy' include U.S. Patent 1005338, 1018555, and 1597901.
- ^ Bell, Louis (1901). Electric Power Transmission; a Practical Treatise for Practical Men, p. 10. Retrieved on 2007-02-15. "Both kinds of strains exist in radiant energy, [...] The stresses in electro-magnetic energy are at right angles both to the electrostatic stresses and to the direction of their motion or flow."
- ^ Anderson, Leland I. (editor), Nikola Tesla On His Work With Alternating Currents and Their Application to Wireless Telegraphy, Telephony and Transmission of Power, 2002, ISBN 1-893817-01-6.
[edit] Patents
- U.S. Patent 0341213 - Transmitting and recording sound by radiant energy - Alexabder Graham Bell (Filed May 3, 1884; Issued May 4, 1886.)
- U.S. Patent 0685957 - Apparatus for the utilization of radiant energy - N. Tesla
- U.S. Patent 0685958 - Method of utilizing of radiant energy - N. Tesla
- U.S. Patent 1005338 - Transmitting apparatus - Harry Shoemaker (Filed Jun 17, 1905; Issued Oct 10, 1911)
- U.S. Patent 1018555 - Signaling by electroradiant energy - C. D. Ehret (Filed Dec 2, 1903)
- U.S. Patent 1523798 - Perception of radiant energy - E. Benson (Filed Apr 13, 1918; Issued Jan 20, 1925.)
- U.S. Patent 1418792 - System for control of moving bodies by radiant energy - John Hays Hammond Jr. (Filed Aug 6, 1914; Issued Jun 6, 1922)
- U.S. Patent 1317883 - Method of generating radiant energy and projecting same through free air for producing heat - William M. Meacham (Filed Apr 12, 1915; Issued Oct 7, 1915.)
- U.S. Patent 1379166 - Radiant energy signalling system case - Theodore Case (Filed Jan 22, 1918; Issued May 24, 1921.)
- U.S. Patent 1425523 - Transmission system for radiant energy - John Hays Hammond Jr. (Filed Jun 22, 1917; Issued Aug 15, 1922)
- U.S. Patent 1424641 - Marine trailer for radiant energy recieving systems - John Hays Hammond Jr. (Filed Dec 23, 1918; Issued Aug 1, 1922)
- U.S. Patent 1597901 - Radio apparatus - Arthur Atwater Kent (Filed Nov 29, 1922; Issued Aug 31, 1926.)
- U.S. Patent 2425102 - Radiant energy reciever - Gilbert C. Larson (Filed Sep 20, 1943; Issued Sep 10, 1846.)
- U.S. Patent 3971938 - Method of generating electricity from radiant energy called variable polarizability capacity generator - L. R. O'Hare
[edit] Other
- Caverly, Donald Philip, "Primer of electronics and radiant energy" New York, McGraw-Hill, 1952.
- Hardis, Jonathan E., "Visibility of Radiant Energy". PDF.
- Lighting Design Knowledgebase
